Aluminum metal is conventionally produced by the electrolytic reduction of alumina dissolved in a molten cryolite bath according to the Hall-Heroult process.
This process for reducing alumina is carried out in a thermally insulated cell or "pot" which contains the alumina-cryolite solution. The cell floor, typically made of a carbonaceous material, provides some of the thermal insulation and serves as a part of the cathode. The cell floor may be made up of a number of carbonaceous blocks bonded together with a carbonaceous cement, or it may be formed using a rammed mixture of finely ground carbonaceous material and pitch. The anode, which usually comprises one or more carbonaceous blocks, is suspended above the cell floor.
Resting on the cell floor is a layer or "pad" of molten aluminum which the bath sees as the true cathode. The anode, which projects down into the bath, is normally spaced from the pad at a distance of 1.5 to 3.0 inches (3.81 to 7.62 centimeters). The alumina-cryolite bath is maintained on top of the pad at a depth of about 8.0 to 12.0 inches (20.32 to 30.48 centimeters).
As the bath is traversed by electric current, alumina is reduced to aluminum at the cathode and carbon is oxidized to its dioxide at the anode. The aluminum thus produced is deposited on the pad and tapped off periodically after it has accumulated.
For the electrolytic process to proceed efficiently, the alumina reduction should occur onto a cathode surface of aluminum and not the bare carbonaceous surface of the cell floor. Therefore, it is considered important for the pad to cover the cell floor completely.
As molten aluminum does not readily wet or spread thin on carbonaceous materials, the pad can best be visualized as a massive globule on the cell floor. In larger cells, the heavy currents of electrolysis give rise to powerful magnetic fields, sometimes causing the pad to be violently stirred and to be piled up in selected areas within the cell. Therefore, the pad must be thick enough so that its movements do not expose the bare surface of the cell floor. Additionally, the anode must be sufficiently spaced from the pad to avoid short circuiting and to minimize reoxidation of aluminum.
Still, the movements of the pad have adverse effects which cannot be readily controlled. For a given cell operating with a particular current of electrolysis, there is an ideal working distance between the cathode and the anode for which the process will be most energy efficient. However, the required spacing of the anode due to the turbulence of the pad prevents this ideal working distance from being utilized. Further, since the pad is in a state of movement, a variable, uneven working distance is presented. This variable working distance can cause uneven wear or consumption of the anode. Pad turbulence can also cause an increase in back reaction or reoxidation at the anode of cathodic products, which lowers cell efficiency. In addition, pad turbulence leads to accelerated bottom liner distortion and degradation through thermal effects and through penetration by the cryolite and its constituents.
It has been suggested in the literature and prior patents that certain special materials, such as refractory hard metals (RHM), most notably titanium diboride (TiB.sub.2) can be used advantageously in forming the cell floor.
Ideally, in contrast to conventional carbon products, these materials are chemically compatable with the electrolytic bath at the high temperatures of cell operation and are also compatible chemically with molten aluminum.
Also, with these special materials, the electrical resistance across the interface between the molten aluminum and the cell floor is much lower than where the cell floor is formed by bare carbon. Thus, it should be possible to operate the cell with reduced electrical power requirements.
Furthermore, the special cell floor materials are wetted by molten aluminum. Accordingly, the usual thick metal pad should no longer be required, and molten aluminum may be maintained on the cell floor as a relatively thin film. This can be conveniently carried out using a "drained cathode" configuration where molten aluminum is continuously drained off the cathode as the aluminum is electrolytically reduced.
By using a drained cathode design, the substitution of a relatively thin film of molten aluminum for the conventional metal pad eliminates a source of electrical resistance in the cell. In addition, the anode-cathode working distance across the bath can be shortened considerably by going to a drained cathode design, which would reduce the electrical resistance of the cell still further, and would also permit the most efficient anode-cathode working distance to be utilized.
With all of their benefits to the reduction process, there is a problem associated with the use of RHM tiles as the reduction cell floor. When attached to carbonaceous substrates, such as the carbonaceous cathode of a reduction cell, errosion occurs at the RHM tile-carbonaceous substrate interface in the presence of molten aluminum and electrolyte. It is believed that this errosion is primarily chemical in nature, with the molten aluminum wetting the tile surface and reacting with the carbon to form Al.sub.4 C.sub.3 which then dissolves in the electrolyte. This sets up a mechanism for removal of carbon from the tile interface and below, causing detachment of the tiles from the cathode.
It is thus a primary object of the present invention to eliminate the cause of this reaction.